Hello fellow summoners this is my guide on Kha'Ziks in the mid lane.
This build is based on max damage output with little points in durability, but if your team losing get some.
IF YOU ARE WINNING LANE DOMINATE YOUR OPPONENT, PLAY SUPER AGGRESSIVE, DENY HIM FARM AND MAKE IT HARD FOR HIM/HER TO COME BACK (warning don't be super aggressive early game becuause towers hit like a mach truck early game and your poke W cost so much mana early gameyou can't spam it).

As i mention above Evolve W => Q => E if you are dominating but get W => Q => R if you dying alot
COMBO => if you are in lane LEAP => VOID SPIKE => TASTE THEIR FEAR then just ignite/spam Q
If you are trying to get away VOID SPIKE for slow => VOID ASSAULT for invisibility => LEAP to get out of their range

You can just look above for item guide.
I have added two builds one is for id you are doing well (good farm getting kills)
The other is for not so well games (denied alot of farm low CS and are gettin killed easily in 1v1s early)

At the start of the game your team wont want to be getting in too much fights. Well not until Lvl 13 when u get your Q and W maxed.This is why i recommend getting W first and when u evolve it your farm gets amazing. Also it gives you some poke. I wouldn't recommend spamming it at the start of the game as the mana cost is pretty high when you start upgrading it.

When you are around Lvl 13 and you are poking your opponent out of you lane then you can gank when he recalls. When enemy is in suitable ganking position use LEAP => VOID SPIKE => VOID ASSAULT => TASTE THEIR FEAR => VOID ASSAULT => NOW JUST AA AND USE Q AND W TO FINISH IF HE/SHE HAS NOT DIED YET. (shouldn't be that tanky to take that long early game so it is alot easier early game)

You are not very tanky when you are taking the brunt of the damage so stay back a little until your tank/ jungler initiate. Use your W as much as possible and if and enemy comes after you try to get him just out of range of his teammates so you get an attack buff of of your Q and if they are a caster and have the range just LEAP to them and W => Q them spam AA and Q's

If you feel your team is losing a fight you can run away with VOID ASSAULT ( IF YOU USE VOID ASSAULT TO RUN INTO BUSH THEN FLASH OVER WALL ENEMIES NEARLY ALWAYS LOSE YOU )

LATE GAME PEOPLE PUSH MID TURRET SO IT IS UP TO YOU TO DESTROY THE MID TOWER SO YOUR TEAM CAN START PUSHING AND ALSO IF YOU ARE DOING GOOD YOU CAN GANK ALOT MORE AND HELP OTHER LANES.

With your insane single target burst late game you can easily carry team and it is hard too lose if you keep killing them :D

Pros
+INSANE SINGLE TARGET BURST
+GOOD MOBILITY
+LEAP IS LIKE AN EXTRA FLASH
+EXTREMELY STRONG GANKER
+DESTROYS LOW HEALTH CASTERS/GLASS CANNONS
+LEAP CAN LET YOU ESCAPE FROM GANKS

Cons
+IF HE GETS SHUT DOWN EARLY GAME IT IS REALLY HARD TOO COME BACK
+NOT VERY GOOD VS PEOPLE WHO BUILD ALOT OF ARMOR
+DIES REALLY QUICK IF HE IS FOCUSED IN TEAMFIGHTS
IF FLASH AND LEAP ON CD HE IS REALLY SUSCEPTIBLE TO GANKS, ESPECIALLY ONES WHO HAVE STUNS,SNARES ,FEARS OR SLOWS)

Though i would add this section too tell you how to farm a wave properly
If their are enemies in lane with you use AA or Q to last hit. Q easier because it does more damage.
Avoid using/spamming W early game as it cost ALOT of mana to use(i think at lvl 1 you can only use it 8 times before you run out of mana so it is better to farm early game with Q)
If you are clearing wave too stop a bunch of stacked minion waves pushing a turret LEAP caster minions and then W the ones that are still alive, finish off melee with Q and AA.